The empty field between the Lewiston–Auburn Water Pollution Control Authority, center, and Lincoln Street, right, seen Friday, will be the site of a large holding tank that will collect sewage and stormwater overflow during heavy rainfall to help prevent it from going into the Androscoggin River. Russ Dillingham/Sun Journal

LEWISTON — When heavy rainfall overwhelms Lewiston-Auburn’s wastewater system, there’s only one place to relieve the pressure: the Androscoggin River. Last year, 52 million gallons of diluted but untreated sewage went into the river.

Despite years of trying to separate the Twin Cities’ stormwater and sewerage systems to address the issue, in some places the systems remain combined. For that reason, heavy rain events – which are expected to be more frequent with climate change – can add millions of gallons of unintended rainwater to the treatment plant’s load, overwhelming the cities’ wastewater facilities.

The Twin Cities are embarking on a project to help come closer to eliminating the contamination. Officials overseeing Lewiston-Auburn’s wastewater treatment plant are accepting bids to build a 2.1 million-gallon wastewater holding tank at the 535 Lincoln St. location in Lewiston.

The project is expected to cost around $25 million. Area and state officials hope that after its projected completion in January 2026, the holding tank will help bring the Lewiston-Auburn wastewater system closer to its goal of never needing to release overflows into the Androscoggin.

Lewiston and Auburn rely on a complex system of pipelines and treatment facilities to move and decontaminate wastewater. Water enters the system primarily as either sewage and wastewater from local households, businesses and schools, or as rainwater from storm drains.

Regarding the sewage and wastewater component, the two cities each have their own smaller treatment programs, but most wastewater flows through underground pipes to the Lewiston-Auburn Water Pollution Control Authority facility, Maine’s second-largest wastewater treatment plant. The plant treats wastewater to eliminate contaminants that would otherwise end up in the Androscoggin.

Over the years, work on the system has separated the pipes carrying rainwater from the pipes carrying sewage and wastewater in many areas of the two cities. But not all.

Because of those still-combined areas, in a heavy rainstorm the treatment plant can’t handle the volume of water coming in. It can treat up to 25 million gallons in a single day at peak capacity, but that’s not enough to cover the heaviest storms, which can send up to 50 million gallons of water to the facility. That excess has to go somewhere.

Currently, when flows threaten to overwhelm the system, the water bypasses the facility and goes directly into the river. But the new holding tank will allow the the treatment plant to store some excess wastewater for treatment later, rather than being forced to release it into the Androscoggin.

“When those pipes fill up, you’ve got to have some relief,” said Travis Peaslee, the treatment plant’s general manager. “Either you’re going to relieve it (into the Androscoggin River) … or you’re going to relieve it in someone’s basement. Obviously, we’d rather be in the (newspaper) talking about tanks and storage than flooding basements.”

Building large holding tanks is a common way for water districts to expand the capacity of their wastewater treatment facilities and limit watershed contamination during heavy rainstorms. Bangor finished constructing a 3.8 million-gallon tank last year, while a 3.5 million-gallon tank is under construction in Portland, visible off Interstate 295 in Back Cove. Lewiston has built three smaller holding tanks over the past 20 years, with a combined capacity of just under 2 million gallons.

Minimizing the environmental damage caused by overflow events has been a priority for the Maine Department of Environmental Protection for over 30 years; the state has spent over $1 billion on upgrading wastewater systems since 1990.

The work has paid dividends: Last year, sewage outflows from Lewiston and Auburn were less than a tenth of what they were in the late 1980s, even as both cities have grown in population, according to state data. But Maine’s goal is to entirely eliminate untreated overflows.

In the long term, completely eliminating overflow events will require work on Lewiston and Auburn’s pipe infrastructure, not just increasing the treatment plant’s holding capacity. Currently, both cities route some sewage and stormwater runoff through the same pipes, which is why wastewater volume spikes after a rainstorm. This is common across New England and in the Great Lakes region, home to the country’s oldest wastewater infrastructure.

Ideally, wastewater and stormwater systems should be completely separated – sewage would travel through its own set of pipes to the treatment plant, while stormwater would be routed into the Androscoggin. Most newer wastewater systems are designed this way. But for the older combined systems common across Maine, pursuing “separation” projects to prevent mixing of sewage and stormwater can be expensive and inconvenient, especially because so much water infrastructure is buried underground, beneath aging roads and buildings.

“Work to eliminate these (overflow events) has been going on since the late 1980s,” said Mike Riley, a senior environmental engineer at the DEP. “When it first started out, everybody wanted to separate their way out of the problem. … What they found over their first 20 years is that it’s very expensive, and it takes way too long to do. And city residents aren’t happy with streets being torn up every year.”

In the past 25 years, Lewiston and Auburn both have made huge strides in separating their systems and minimizing overflow events. In 1999, 31% of Lewiston’s wastewater systems were separated. Today, it’s 96%. In 1999, the Lewiston wastewater system discharged raw waste into the Androscoggin on 32 occasions. Last year, it did so eight times. But getting down to zero will require a lot more work.

“We’ve got a lot of very old infrastructure to work around – we’re talking pipes from the 1800s,” said Jeff Beaule, who has been Lewiston’s overflow project engineer since 1999. “The downtown area is the oldest combined area. There are telephone ducts and electrical ducts and old infrastructure and a lot of stuff that we don’t even know about that we find as we’re digging. … And there are also a lot of private drains that are really hard to get to and separate without going onto private property.”

Building holding tanks in order to increase effective treatment capacity is a less disruptive way to decrease overflow events. The cost of the new holding tank project at the treatment plant will be shared by both Lewiston and Auburn. In terms of upfront costs, Lewiston is contributing $9.5 million sourced from a COVID-19 stimulus grant, and Auburn sold $4 million in municipal bonds to cover its share. The remaining costs, likely between $10 million and $15 million, will be covered by a low-interest, 20-year loan from the state.

Based on an independent analysis, Lewiston will cover 70% of the initial cost of the project, and Auburn will cover the remaining 30%. That split is based on historical water usage, but it’s designed to change over time.

“Once we’re actually operating, (that 70-30 split) will be reevaluated on a year-to-year basis,” Peaslee said. “Over time, it’s going to shift. If their populations grow and the flows grow more, (or) if they continue to do more separation projects and set less flow … that could change things down the line.”

Climate change is one factor working against Maine’s efforts to combat overflow events that contaminate watersheds across the state, making it more challenging for the DEP to achieve its goal of eliminating all overflow events in Maine.

“Climate change is causing issues with intensity and sometimes the duration of storms,” said Riley, the DEP engineer. “Sewer systems are designed to carry flows of so many gallons, and when you put in more than that with a big storm, they don’t respond well. That’s been a big issue in all the past five years. We’re seeing these storms that no sewer system is designed to handle.”